On the recent mobile phone market, models of mobile phones having built-in camera modules have become dominant. These camera modules need to be embedded in the mobile phones. Therefore, the camera modules face greater demands for being compact and lightweight, as compared with camera modules to be embedded in digital cameras.
Among such camera modules, there have been an increasing number of camera modules that (i) achieve their autofocus (AF) functions with the use of lens drive devices and (ii) are mounted on electronic devices such as mobile phones. Various types of lens drive devices have been developed so far, and examples of the lens drive devices encompass those employing stepper motors, those employing piezoelectric sensors, and those employing Voice Coil Motors (VCM). Such lens drive devices have already been distributed on the market.
Meanwhile, in such a circumstance that the camera module having the autofocus (AF) function has become standard, the image stabilization function has been attracting attention as a next distinctive feature for camera modules. Although the image stabilization function is widely employed in digital cameras and camcorders, there have been only a few mobile phones employing the image stabilization function, due to their limited sizes. Nevertheless, mobile-phone-specified camera modules having the image stabilization function are expected to be mainstream in the future, and, in fact, there have been an increasing number of proposals on a novel structure of an image stabilization system which achieves reduction in size.
As for the camera module having the autofocus function, Patent Literature 1 discloses a highly potential technique for achieving reduction in size and a small tilt. Patent Literature 1 has proposed a technique for eliminating a screw structure for focus adjustment, which screw structure is provided both in the lens barrel and in the lens holder, to omit a space for a screw thread.
Specifically, a camera module disclosed in Patent Literature 1 is, as illustrated in FIG. 10, a camera module 100 having the AF function. The camera module 100 includes (i) an optical section 101, (ii) a lens drive device 102 for driving the optical section 101 in an optical axis direction, and (iii) an image pickup section 103 on which the lens drive device 102 is mounted. The image pickup section 103 is constituted by a lamination of a sensor cover section 104 and a substrate 105. The optical section 101 is constituted by a plurality of image pickup lenses 106 and a lens barrel 107 holding the plurality of image pickup lenses 106. The lens drive device 102 includes (i) a lens holder 108 holding the lens barrel 107 and being driven in the optical axis direction of the optical section 101 (ii) an upper leaf spring 109a and a lower leaf spring 109b supporting the lens holder 108 so as to be movable in the optical axis direction of the optical section 101, (iii) an AF coil 110 integrally fixed to the lens holder 108, (iv) a yoke 111 constituting a fixing section, (v) a magnet 112, fixed to an inner wall of the yoke 111, generating an electromagnetic force between itself and the AF coil 110, (vi) a cover 114, provided on an upper surface side of the yoke 111, for protecting the upper leaf spring 109a, and (vii) a base 115, provided on a bottom surface side of the yoke 11, supporting the whole of the lens drive device 102. The sensor cover section 104 includes an infrared ray (IR) cut filter 120, an image pickup element 121, and a sensor cover 122. The image pickup element 121 is mounted on the substrate 105.
In the camera module 100 disclosed in Patent Literature 1, an outer periphery of the lens barrel 107 and an inner periphery of the lens holder 108 are not threaded. Further, the camera module 100 has a structure eliminating the need for focus adjustment, i.e. the structure that the lens barrel 107 is positioned by being brought into contact with on the upper surface side of the sensor cover 122. This enables the camera module to become smaller in size by a screw thread.
Furthermore, in the camera module 100 disclosed in Patent Literature 1, the outer periphery of the lens barrel 107 and the inner periphery of the lens holder 108 are arranged such that the lens barrel 107 and the lens holder 108 each have the same diameter across their whole areas. In other words, the lens barrel 107 and the lens holder 108 are formed in a flat cylindrical shape.
The camera module 100 disclosed in Patent Literature 1 has the autofocus function. However, the focus-adjustment-less structure is applicable to a camera module which has an image stabilization function as well as the autofocus function. Patent Literature 2 discloses a focus-adjustment-less structure of a camera module having both the auto focus function and the image stabilization function. Patent Literature 2 further discloses a step structure having a maximum outer diameter section formed in a belt shape, instead of having a lens barrel being flat in outer shape and being formed in a cylindrical shape.
Note that, as used herein, the “maximum outer diameter section (formed in a belt shape)” of the lens barrel has certain thicknesses in a height direction of the lens barrel (in an optical axis direction of the optical section) and in a width direction of the lens barrel (in a direction vertical to the optical axis of the optical section), and is a protrusion provided so as to surround a side surface of the lens barrel. The maximum outer diameter section has the largest outer diameter in the lens barrel in the width direction of the lens barrel.
The following discusses a meaning of having the step structure in the outer shape of the lens barrel as disclosed in Patent Literature 2, with reference to (a) and (b) of FIG. 11.
The camera module disclosed in Patent Literature 2 includes a lens barrel 151 having (i) a maximum outer diameter section provided partially in the lens barrel 151 rather than entirely in the lens barrel 151 and (ii) the other sections having slightly smaller diameters (see (a) of FIG. 11 illustrating a sectional view of a main part of the camera module). More specifically, the camera module illustrated in (a) of FIG. 11 includes the lens barrel 151 having a maximum outer diameter section 151a formed in a belt shape. Here, an angle of inclination θ of the lens holder 152 with respect to a surface of a sensor cover (a dummy sensor cover 153) is expressed as a difference between θ1 and θ2 in (b) of FIG. 11 and is set to satisfy the following expression:θ≦tan−1(H/DE)−cos−1(D1/√(DE2+H2))wherein DE is an outer diameter of the maximum outer diameter section 151a of the lens barrel 151, H is a thickness of the maximum outer diameter section 151a, and D1 is a cylindrical inner diameter of the lens holder 152.
Hence, by controlling an inclination (tilt) of the cylindrical hole of the lens holder 152 to within a predetermined range when the lens barrel 151 is inserted into the lens holder 152 while the lens barrel 151 is being slid within the cylindrical hole of the lens holder 152, installation of the lens barrel 151 can be performed with reference to the sensor cover (dummy sensor cover 153), without being influenced by an inclination of the cylindrical hole of the lens barrel 151.
In other words, in a case where the value H becomes too large, a tolerance value of the tilt of the lens holder 152 becomes smaller. As a result, it becomes difficult to fix the lens holder 152 with a high accuracy.